fix(ios/travis-ci) try pod update

With the fastimage lib Travis complains about:

CocoaPods could not find compatible versions for pod "SDWebImage/Core"

ios/travis-ci/build-ipa.sh

@@ -130,6 +130,7 @@ cp "${CERT_DIR}/dev-profile.mobileprovision"  ~/Library/MobileDevice/Provisionin
 npm install
 
 cd ios
+pod update
 pod install
 cd ..